The clustering of neurotransmitter receptors at the post-synaptic membrane of synapses is essential for efficient transmission of a signal from a neuron to its target. At the cholinergic mammalian neuromuscular junction (NMJ) the mechanisms of acetylcholine receptor clustering and the physiology of these neurotransmitter receptors have been well studied. However, little is known about the roles of other receptors and channels, including NMDA glutamate and ErbB4 receptors, and sodium channels that are also localized to this post-synaptic membrane. A family of proteins containing a PDZ protein motif localizes channels and receptors at glutamatergic Drosophila NMJ and mammalian central nervous system synapses and are thought to play a fundamental role in the organization, structure, and function of these synapses. We have recently shown the localization of several of these PDZ domain proteins at the post-synaptic membrane of the mammalian NMJ. Two of these PDZ domain proteins, Dlg and CASK, represent excellent candidates for interacting with the NMDA and ErbB4 receptors and sodium channels at the NMJ post-synaptic membrane. The aims of the current application are to determine the proteins with which Dlg and CASK interact in skeletal muscle, whether these two proteins are responsible for localizing channels and receptors at the NMJ, and whether mutations in these proteins lead to alterations in the function of this synapse. These studies will utilize immunological and biochemical fractionation techniques together with myogenic cell culture to identify binding partners for Dlg and CASK and to determine the domains responsible for their localization and protein interactions in skeletal muscle. Transgenic mice will be used to define the functions of Dlg and CASK and their PDZ domains in channel and receptor clustering in skeletal muscle in vivo and provide valuable information about the effect of these receptors on the physiological characteristics of muscle. These studies will lead to important new insights into the roles of PDZ domain proteins in the organization and function of the NMJ and will have important implications for understanding a wide variety of neuromuscular diseases including channelopathies and those disorders for which a cause is unknown.